It is well known thatdislocations in FCC metals are composed of partial dislocations separated bystacking faults. When consideringthe reactions of dislocations with each other, such as in DD simulations, it is necessary to determine therelative positions of the partials in order to correctly describe theconfigurations that are created in the reactions. Here we describe a geometric method for correctly determiningthe relative positions of the partials. The results we obtain can also be found by applying an axiom, or rule,given in the book by Hirth and Lothe. At the end of this tutorial we describe this axiom and apply it to someof the problems we have discussed. Our hope is that the geometric method we describe will give readers anintuitive sense of the geometry of partial dislocations and confidence in usingthe Hirth/Lothe axiom.